Optimise the use case where the two conditions overlap in an =

[nterbogt]

There is a special use case where the depth and the start overlap and create SQL that looks like (a <= x) & (a >= x)... this is equivalent to (a=x) and as a result allows better use of the table indexes, specifically the (id, revision_id, left_pos, right_pos, depth).

Before optimisation:

id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	parent	const	PRIMARY,IDX_E1907B87BF3967501DFA7C8F5E183DDBDEADD74FAA31C69,IDX_E1907B875E183DDBDEADD74,IDX_E1907B87BDEADD74	PRIMARY	8	const,const	1
1	SIMPLE	child	range	IDX_E1907B875E183DDBDEADD74,IDX_E1907B87BDEADD74,IDX_E1907B87FAA31C69	IDX_E1907B87BDEADD74	20	NULL	145780	Using index condition; Using where; Using filesort

After optimisation:

id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	parent	const	PRIMARY,IDX_E1907B87BF3967501DFA7C8F5E183DDBDEADD74FAA31C69,IDX_E1907B875E183DDBDEADD74,IDX_E1907B87BDEADD74	PRIMARY	8	const,const	1
1	SIMPLE	child	ref,filter	IDX_E1907B875E183DDBDEADD74,IDX_E1907B87BDEADD74,IDX_E1907B87FAA31C69	IDX_E1907B87FAA31C69,IDX_E1907B87BDEADD74	4,4	const	12148	Using index condition; Using where; Using filesort; Using rowid filter

[nterbogt]

All tests passed on this, but there might be a slight flaw in the math. Putting in Draft until I can review.